75424-Neutral species play important roles in many aspects of the operation of tokamak fusion reactors, including divertor operation, particle recycling, and edge turbulence effects. Diagnostics are required to quantitatively determine the state of neutral species, including their densities, temperatures, and directed velocities. However, current diagnostics are difficult to absolutely calibrate and suffer from the lack of spatial resolution. This project will develop a resonant, heterodyne interferometer to measure chordal, ground state densities of various neutral transitions including H(, D(, and T(. In Phase I, the interferometer will be designed, fabricated, and tested with in-house plasma sources. The critical issues are to demonstrate device operation with a tunable laser source, to optimize immunity to environmental noise (i.e., vibrational effects), and to demonstrate a capability to make meaningful measurements in plasmas akin to tokamak plasmas. In Phase II, the instrument will be installed and operated on the Alcator Cmod tokamak. Commercial Applications and Other Benefits as described by the awardee: In addition to the fusion energy application, the resonant interferometer should be robust enough for an industrial setting and would be an ideal sensor to monitor and control the plasma deposition of metal/diamond-like-carbon coatings, used to reduce friction, reduce wear, extend service life and improve the reliability of automotive components.